Cellulose is a natural polymer that has been used for centuries to make various forms of paper. More recently, it has been chemically regenerated to make fibers, transparent paper, cellophane, films, and casings for industrial and food related uses. These cellulose-based casings are well known for use in food products, such as sausages. Cellulose fibers have also been treated and reformed as webbed paper products where the individual cellulose fibers are physically brought together, for example, by needle punching, and form products such as non-woven sheets and towels.
The regenerated cellulose polymer used for making both casings and films is most commonly produced by the well known viscose process. A natural cellulose, such as wood pulp or cotton linters, is treated with a caustic solution to activate the cellulose to permit derivatization and extract certain alkali soluble fractions from the natural cellulose. The resulting alkali cellulose is shredded, aged, and treated with carbon disulfide to form cellulose xanthate, a cellulose derivative. The cellulose xanthate is then dissolved in a weak caustic solution. The resulting solution, or viscose, is ripened, filtered, deaerated and extruded as a film or tube into coagulation and regenerating baths containing salts and sulfuric acid. In these acidic baths, the cellulose xanthate, e.g., viscose, is decomposed and hydrolyzed back to a pure form of cellulose. A drawback of this method is that undesirable side products, including sulfur, carbon disulfide and hydrogen sulfide, are produced.
Although the viscose process is the most commonly used process for the production of cellulose casing for the food processing industry, alternate methods have been described, in particular that described in U.S. Pat. No. 5,277,857. This alternate cellulose production method involves forming a polymeric solution by means of a simple dissolution of cellulose, rather than derivatization, to form a soluble substance. The cellulose dissolution process is described in U.S. Pat. No. 2,179,181. This patent describes the dissolution of natural cellulose by a tertiary amine oxide to produce solutions of relatively low solids content, for example, 7 to 10% by weight cellulose dissolved in 93 to 90% by weight of the tertiary amine. The cellulose in the resulting solution is nonderivatized prior to dissolution. U.S. Pat. No. 3,447,939 discloses use of N-methylmorpholine-N-oxide ("NMMO") as the amine oxide solvent of choice.
More recent patents, such as, U.S. Pat. No. 4,145,532 and U.S. Pat. No. 4,426,288, improve upon the teachings of the '939 patent. U.S. Pat. No. 4,145,532 discloses a process for making a solution of cellulose in a tertiary amine oxide, such as NMMO, that contains 10-35% by weight of cellulose. This higher solids content, achieved in part by including an amount of water (from 1.4% to about 29% by weight) in the tertiary amine oxide solvent, provides a solution adapted for shaping into a cellulosic article by extrusion into fibers, films, or casings. In U.S. Pat. No. 4,426,288, the NMMO-cellulose polymeric solution contains an additive that reduces decomposition of the cellulose polymer chain so that molding or spinning substances are obtained with only slight discoloration and that will yield molded shapes distinguished by improved strengths upon regeneration in a nonsolvent such as water.
Using NMMO as a solvent for cellulose eliminates the disadvantages attendant to the viscose process including the generation of toxic and noxious gases and sulfur compounds.
Cellulose food casings generally are made in a nonreinforced or a reinforced form. Nonreinforced casing consists of a tubular film of regenerated cellulose polymer having a wall thickness ranging from about 0.025 mm to about 0.076 mm and made in diameters of about 14.5 mm to about 203 mm.
Reinforced casings are commonly called "fibrous" casings to distinguish them from the nonreinforced cellulose casings. Here, a lighter viscosity viscose than is used for nonfibrous casing is extruded onto a sheet or tube of paper prior to entering the coagulation and regenerating baths. The result is that the regenerated cellulose impregnates and bonds to the paper substrate. Fibrous casings have a wall thickness in the range of 0.050 mm to 0.102 mm and are made in diameters of about 40.6 mm to 193 mm or greater. Fibrous casings have a dimensional stability that is greater than that found in nonreinforced casing. These casings are known for having enhanced wet strength and resistance to puncture and propagation of tears. As fibrous casings are stiffer and are less extensible than nonreinforced casings, they assure that once the casing is stuffed with meat each linear inch of casing will contain a specific weight of meat. This enables the sausage manufacturer to easily prepackage weights of sausage meat by number of slices instead of by weight. To the best of applicants knowledge, the method of making casing using the amine oxide solution as described in U.S. Pat. No. 5,277,857 has not been extended to making fibrous casings.
Another type of casing using "filled" systems that are well known in the art are collagen-based casings. Filled systems can be described as being composed of a basic material, such as collagen, that has another material added to it that never chemically bonds or becomes part of the first basic material. Often this system is used in order to improve the economics of the product by using a cheaper "filler" material with the more expensive basic material. The added material, up to a critical amount, may be able to add its properties to those of the basic material. However, when too much of the added material is present, it interferes with the properties of the basic material.
An example of such a system is polyvinylchloride ("PVC") polymer containing an additive to render pipe made from the PVC polymer resistant to degradation by sunlight. A small amount of the additive neither adds nor takes away from the strength of PVC pipe and additionally protects the pipe from degradation by sunlight. However, a large amount causes the PVC pipe to degrade as the continuity of the polymer is interrupted by the sheer amount of the protective additive.
In order to impart certain strength and handling characteristics to collagen casings, other non-collagen materials, such as, cellulose, acids, bases, salts, and rayon fibers have been added to the collagen prior to forming it into a casing. U.S. Pat. No. 4,378,017 describes and teaches a composite material of de-N-acetylated chitin and fibrous collagen that has excellent mechanical strength and heat-resistance, and which can be used as casing. U.S. Pat. No. 4,061,787 describes a collagen casing with improved strength characteristics containing a crosslinking agent that is a fatty acid or oil and the collagen.
An earlier U.S. Pat. No. 3,551,535, describes a method for making a homogeneous mixture of collagen, reactive materials and shrink control agents. Reactive materials are those that show chemical or physical action when brought together, such as, the swelling of collagen when in contact with an acid, base, or salt. Shrink control agents include cellulose fibers, rayon fibers, cotton fibers, alginates and starches. The compounds formed can be used for articles, such as, casings, shoes, gloves, medical sutures, and bandages.
Although use of a filled system can produce a collagen casing having attractive properties of strength, it has not been used with cellulosic casings.
By contrast, composite technology can be used to create composites where successive incremental additions of an added material to a high percentage of the overall composite do not cause the degradation of any salient property of these compositions. A "composite" is formed when a second material is added to a first material, such as a polymer, and not only does the added material not disrupt the integrity of the original polymer and the article formed from it, but can add its own characteristics to the polymer.
However, up until the present time, the only way to achieve improved properties of casings made from viscose has been by coating or bonding the viscose to a paper web. Although fibrous casing is generally satisfactory for the casing user, there are disadvantages for both the user and the manufacturer. Disadvantages for the casing manufacturer include the need to maintain a supply of paper web or sheet in addition to the viscose used for nonreinforced casing, supplier problems, running problems on the coating machines, and the need to make a uniform seam when the paper web is seamed. This seam must be uniform and cannot break apart when the casing is manufactured and stuffed with food under high pressure. The paper is generally a specialty item, hemp paper, and obtainable only from a limited number of suppliers. Differential tensions between the paper and the regenerated cellulose coating can cause tension lines, wrinkles, and other discontinuities in the casing.
Therefore, there is a need in the art to have a casing with the properties of fibrous casing but manufactured without a paper tube, especially if such casing could be made without any noxious side products, and preferably by use of a non-viscose process.